Refining of copper



Patented June 14, 1932 WALTER O. Sll'EZ-LING, OF ALLENTOWN, PENNSYLVANIA REFINING OF COPPER No Drawing.

My invention relates to improvements in the refining of copper and more particularly relates to improvements in the refining operation technically known as fining or flapping. One of the objects of my present invention is to provide means for the more cilicient oxidation of the impurities present in copper during the fining or flapping operation, and thereby securing the more complete elimination of such impurities. Another object of my invention is to lessen the cost of the refining of copper, through materially reducing the necessary time of the fining operation and of the subsequent poling operation, thereby not only increasing the refining capacity of the refining furnace but very greatly reducing the fuel cost of the entire refining operation.

The purpose of the refining operation is to eliminate from copper such oxidizable impurities as sulfur, selenium and tellurium, arsenic, antimony, bismuth and sometimes also iron, cobalt, nickel, lead, tin, etc. The chemistry of the operation is relatively simple. The copper is melted in furnace which is usually of the reverberatory type,

and air is then passed over the surface of the molten copper, and sometimes bubbled through the molten copper by means of air pipes inserted into the bath, until a notable percentage of the copper has been oxidized to cuprous oxide. In commercial practice the oxidation of the copper is continued until approximately 0.5% to 0.8% of oxygen is present in combined condition in the molten copper, corresponding to the presence of from 4.5% to 7.0% of cuprous oxide in the copper. The cuprous oxide thus present in the molten copper reacts with the oxidizable impurities which are also present in the copper, and results in the elimination of the impurities, sulfur for example being oxidized to sulfur dioxide and passing off as a; gas, and many of the other impurities being first oxidized and then eliminated from the copper as slag. At the close of the fining operation the copper contains from 6.0% to 7.0% of cuprous oxide, which must of course be eliminated, and this is brought about in present commercial practice by an operation known as poling, and

Application filed December 30, 1930. Serial No. 505,587.

which consists essentially in covering the surface of the molten copper with a layer of charcoal or coke and then plunging beneath the surface of the molten copperpoles of green wood which under the influence of the intense heat of the copper bath are rapidly reduced to charcoal and other products of the pyrolysis or thermolysis of the wood. The action of the coke floating on the copper bath and the charcoal and other carbonaceous decomposition products of the wood within the bath rapidly reduces the copper oxide to me tallic copper, and the operation is assisted by the violent stirring of the copper bath which results from the uprising bubbles of water vapor and carbonaceous decomposition prod nets of the wood.

' It will accordingly be seen that the fire refining of copper consists in its essential elements of passing air over or through the molten copper, to form approximately from 5.0% to 7.0% of cuprous oxide, permitting the cuprous oxide thus formed to react with the oxidizable impurities in the molten copper, and then reducing the cuprous oxide back to copper by means of a reducing agent such as coke and the pyrolytic decomposition prodnets of wood.

Because of the relative simplicity of the chemical operations involved in the fining and poling of copper the possibility of improving these operations has apparently been overlooked, and at the present day both the fining'and the poling of copper are carried out in a manner that is exceedingly primitive as compared with many other metallurgical operations, and which is exceptionally wasteful of fuel.

In the commercial refining of copper the reverberatory furnaces which are employed may have hearths from 80 feet to 60 feet in length and from 14 feet to 18 feet in width, and the depth of the hearth may vary from 1.5 inches to 40 inches, such reverberatory furnaces having a capacity under present operating conditions of from 150 tons to 300 tons of copper per 24 hours, the present operating cycle being in general such that one complete fining and poling operation is conducted in approximately 2-1 hours, the exact time being of course somewhat variable, since the fining operation is continued until the oxidation of the copper has gone on sufficiently to result in the presence of about 0.7% of oxygen in combined condition cuprous oxide, corresponding to the presence of about 6.5% of cuprous oxide in the molten copper, and the poling operation being continued until this very great excess of oxygen and cuprous oxide has been reduced until not more than five one-hundredths of one percent of oxygen is still present in the copper.

In order o clearly explain the increasein efliciency which results from the practice of my present invention, and the quite remarkable decrease in the use of fuel which is thereby efiected, it is desirable to explain the operation of fining or flapping as it is now conducted, in order that the distinction between the present practice and my improved method of fining may be more clearly understood.

Taking as an example a refining furnace having a capacity of 300 tons of copper at a charge, the charge of copper is first introduced and the heating of the furnace is then continued for a period of several hours, until the charge is completely melted. The fusing point of copper is 1083 C. or 1980 E,

and accordingly during the entire refining operation the interior of the furnace must be maintained at a temperature in excess of this very high heat. As soon as the charge of copper in the furnace has been melted the oxidation of the copperis commenced. The

side doors of the furnace and the skimming door are fully opened, and the flapping of the charge is commenced. The head of a rabble is repeatedly struck against the surface of the metal to-force back thethin layer of slag which may cover it, and to project some of the copper into the air so as to secure increased contact with oxygen. This operation is continued, for a period of from three to six hours, during which time approximately 500 square feet of surface of molten cop-v per at a temperature in excess of 1100 C. or 2012 F. and in excess of 1,000 square feet of incandescent furnace wall and roof area is exposed to the inrush of air from the fully opened side doors and skimming door of the furnace. To maintain the temperature of the furnace during the fining operation there must be burned from 15,000 pounds to 20,000

c pounds of coal, which performs no useful function in the refining operation beyond maintaining the temperature of the furnace above the melting point of copper during the violent inrush of air which must be maintained for several hours to secure the oxidation of the copper. Although this enormous consumption of fuel performs no useful metallurgical function, it is entirely true that it has very undesirable effects beyond the cost of the. fuel thus wasted, due to the fact that a large part of the sulfur and ash present in the coal thus wastefully burned tends to contaminate the charge of copper being fined, and the sulfur thus added must be removed and the. impurities present in the ash of the 1 coal must be slagged ofi".

lVhen it is remembered that the total amount of oxygen which it is desired to add to the copper is only from 12 pounds to 15 pounds per ton of copper being fined, the extraordinary wastefulness of the present operation will be better understood. For a period of more than three hours great volumes of cold air rush into the furnace through the fully opened side doors and through the skimming door, to be heated within the furnace to a temperature in excess of 1,000 C. or 1832 F. and then discharged through the stack without having performed more thanan infinitesimal portion of the oxidizing effect of which this air is capable. Meanwhile laborers are operating the rabble near the skimming door in the effort to expose a small area of the copper each instant to the direct oxidizing effect of the inrushing air, and in addition to this in recent years compressed air has also been illtroduced through iron pipes the ends of which have been immersed in he molten copper.

A the end of from two to five hours the oxygen content of the copper in the furnace can be increased by the methods described to the desired range of from 0.6% to 0.8% but the operation, as will be noted from the above description is extremely wasteful of time. of labor and. of fuel. t represents the only method of fining known today, however, and is employed in all refining operations of copper in every part of the world.

I have discovered means by which the fining of copper can be completed in about half the time now required in the most modern copper refining furnaces, and with the expenditure of less than half of the fuel now required. My invention also entirely eliminates the present manual or mechanical flapping of the charge, a nd affords means by which the entire fining operation can be conducted in a more rational and efiicient manthat-which is used in copper refining, so that it would be possible tomaterially increase the rate of oxidation of the copper in present refining furnaces if it were possible to maintain the copper at a somewhat higher temper ature than is now employed. This is not possible, however, in view of the vast volnines of cold air which it is found necessary to introduce in the refining furnace, in the operation of refining as now conducted.

I have also discovered the surprising fact that although the introduction of a few tenths of one percent of oxygen into copper is slow and diflicult, the introduction of additional oxygen into copper, after the introduction of the first few tenths of one percent, occurs much more readily and much more rapidly. Although I do not wish to limit myself to any particular theory of operation of my improved process, it is my belief that cuprous oxide acts as a catalyst in the transference of copper to oxygen. After copper has been oxidized until there is present in the molten solution aquantity of cuprous oxide in excess of 3.0%, for example, the addition of further oxygen goes on much more readily, part of the cuprous oxide being apparently over-oxidized to form cupric oxide or some intermediate oxide, and this compound then sharing its oxygen with adjacent copper molecules to form additional cuprous oxide, and being itself thereby reduced to cuprous oxide. This action of course may be repeated over and over, the cuprous oxide taking up oxygen to form amore highly oxygenated oxide of copper and then losing part of its oxygen to produce additional cuprous oxide and itself being thereby brought into condition in which both it and the cuprous oxide thus formed can again pass through the same cycle with the production of still further quantities of cuprous oxide.

1 have discovered that the most important of the several factors which determine the relative sluggishness of the reaction between molten copper and atmospheric air is the partial pressure of oxygen present in air, and I have discovered the very remarkable fact that a relatively slight upward change in the partial pressure of oxygen present in a mixture of oxygen and nitrogen has an entirely disproportionate efiect in bringing about the oxidation of copper. Although a mixture of oxygen and nitrogen in the proportions of 20 parts by volume of oxygen and 80 parts by volume of nitrogen, which is approximately the proportions in which these two gases are present in atmospheric air, reactsvery slow- 1y with molten copper at a temperature of 1100 C., any increase in the percentage of oxygen beyond 25% by volume shows an oxidizing eificiency in the oxidation of copper to cuprous oxide entirely out of proportion to the actual increase in the oxygen present. My experiments have shown that a mixture of 10 parts of oxygen by volume and 90 parts of nitrogen by volume represents a mixture having too low a partial pressure of oxygen to overcome the characteristic inertness of copper to oxidation which is a well known characteristic of the metal, and which gives it much of its usefulness as a non-tarnishing or slowly tarnishing metal at ordinary temperatures, and I have found that even at elevated temperatures the effect of oxygen when present in low partial pressures is exceptionally small, even over a relatively wide range of temperatures. Increasing the partial pressure of oxygen to the point represented by a mixture containing 20 parts by volume of oxygen and 80 parts by volume of nitrogen gives a mixture which is still remarkably unreactive toward copper at the temperature of the melting point of copper, although increasing rapidly in reactivity with temperatures higher than 1100 C. When we make a further step in increasing the partial pressure of the oxygen in a mixture of oxygen and nitrogen, as for example by the use of a mixture of 30 parts by volume of oxygen and 70 parts by volume of nitrogen, an exceptional increase in the reactivity of the oxygen begins to show itself, this increased reactivity manifesting itself not only by greatly increased oxidation at the temperature of 1200 C. and higher, at which the 20-8O mixture shows its most notable oxidizing results, but by a substantial lowering of the temperature necessary to bring about accelerated oxidation, so that relatively rapid oxidation occurs within the temperature range from the melting point of copper, which is generally taken as 108?) (1., and the working temperature of a copper refining furnace which may be taken as approximately 1lOO C. to 1150 C. Still further increasing the partial pressure of oxygen leads to a still further steepening of the oxidation curve, this accelerated action being so great that in some of my experiments I have obtained the oxidation of copper at a rate more than 30 times the rate that was proportional to the actual increase in the amount of oxygen supplied, corresponding to an illcreased rate of oxidation entirely out of proportion to the increased partial pressure of he oxygen present in the mixture, and also entirely out of proportion to the increased amount of oxygen bubbled through the copper in the gaseous mixture.

It will be noted that my work has indicated that the relatively sluggish oxidation of copper in the present fining operation of copper refining may be increased in three entirely V controllable ways, as follows:

1. Increasing the temperature of the molten copper during the fining operation, so as to bring this temperature up to the minimum range in which accelerated oxidation occurs.

2. Increasing the percentage of cuprous oxide in the molten copper so as to obtain advantage of the catalytic efficiency of a high Rea percentage of cuprous oxide in the fixation of. the oxygen present in atmospheric air;

3. Increasing the partial pressure of the oxygen in a mixture of oxygen: and nitrogen supplied to the molten copper to a point sufiicient to pass through the relativelyneutral;

represented by such sluggishly oxidizing mixtures as atmospheric air and similar mix.- tures of. oxygen and nitrogen containing less than 25% by volume of oxygen.

In my pending application S. N. 504,222,v

filed December 22, 1930, I have described and claimed means by which molten copper can be efficiently oxidized in the fining opera-- tion by the use of atmospheric air alone, by suitably combining the increased oxidizing effect of atmospheric air at temperaturesin excess of the maximum temperatures which. are permissible in a fining furnace, with the catalytic activity of a high. percentage of cuprous oxide in bringing about, the absorp tion of oxygen from a mixture in which it is present in relatively low partial pressure and the transferof oxygen so fixed to copper by the action of cuprous and cupric oxides, aided perhaps by intermediate oxides.

My present invention relates to means for improving the fining ofcopper by the use of a gaseous reacting mixture containing oxygen in excess of. the maximum percentage in which oxygen is present in atmospheric air, whereby I am able to efliciently bring about the oxidation of the impurities present in the molten copper without the use of temperatures so high as to be damaging to the furnace hearth, walls or'roof, and without the employment of secondary or separate furnace for the production of cuprous oxide. By my present invention I am ableto bring about a very marked saving'in the time of fining copper, and a: corresponding marked decrease in the cost of fining, through a reduction in the amount of fuel required. By suitable modification of my improved fining process I also obtain increased purification of the copper being fined, as will. hereinafter be more fully described.

In the practice of my present invention I prefer to introduce a gaseous mixture containing more than 25% of oxygen directly into molten copper contained in a fining furnace, preferably employing for this purpose a non-metallic tube which will reach nearly to the bottom of the molten copper,.the tube being preferably tipped at its lowest end with a porous distributing or comminuting molten copper without danger of breakage.

I find that both of these required characteristics are possessed by fused silica, and by en'n ployingt a heavy tube of commercial opaque fused. silica. tipped with a porous end of fritted granular silica particles, I can obtain. the introduction of gaseous mixtures containing high; partial pressures of. oxygen. into molten copper without difficulty, pref erably employing a gaseous pressure barely suificient to overcome the hydrostatic pressure of the molten copper andv the resistance offered to: the'fiow of fluid through the conducting tube and the porous tip, so as to avoid the rapid discharge of the gaseous reacting mixture into the molten. copper, and;

preferably discharging the gaseous reacting mixtune into; the molten: copper as a series of minute bubbles, which condition I find may be produced by the employment of a suit-able tip of fritted finely granulated silica, so as toobtain the necessary small pore spaces which are desirable to produce bubblesof minimum diameter. Instead of a tube of silica I may employ other refractory materials with equal success, however, and I have obtainedentirely satisfactory results by the use of a tube of alund'um or fused aluminum oxide, and satisfactory results may also be obtained by the use of tubes of other materials such as mullite or other high-fusing refractory composed essentially of materials already saturated toward oxygen and hence incapable of further oxidation at the temperature of molten copper.

In the practice of my invention I prefer todepart from the customary procedure of opening all of the doors of the refining furnace during the fining process, as I find that the eifectof opening the doors is mainly to cool off the furnace with great waste of fuel necessary to maintain the charge of molten copper at the fining temperature. Instead of attempting. to maintain a highly oxidizing condition in the atmosphere of the fining furnace above the molten copper I find that actually improved results are obtained by keeping the furnace doors closed and by decreasing; the amount of air sent into the furnace: with. the atomized or powdered fuel (when oilor powdered coal are burned) or by cutting; down the furnace draft, where solid fuel is-burned in a separate grate. As this 'procedure seems strictly illogical when considered in thelight of present fining practicc, which aims to. introduce as much air as possible into the fining furnace above the molten copper, I will attempt to explain the reason for the improved results which I obtain. By introducing as little as 5 pounds of oxygen per ton of molten metal fined, in the form of a gaseous mixture containing a partial pr ure of from 25% to of oxygen, I obtam intense oxidizing effects localized at the point or points at which the gaseous oxidizin mixture is introduced, and pro ducing at these points local concentration of cuprous oxico f r in excess of the desired concentration corresponding to 0.7% of oxygen in the bath considered as whole. As a secondary effect of the intense local overheating at the point of emission of my oxygen-rich gaseous reacting mixture and the collateral stirring effect of any bubbles of nitrogen which are simultaneously discharged, there occurs a circulation of the charge of molten copper forming the bath, and a rapid reduction in the localized high concentration of cuprous oxide that is formed. I find that when copper is alternately somewhat over-oxidized and then partially reduced, the cycle being repeated several times, the elimination of impurities is much more complete than when only a single cycle occurs, as is the case in present fining practice. By my improved method, preferably intro-- ducing air enriched with oxygen through tubes inserted in the furnace through the skimming door, while maintaining slight reducing conditions in the opposite end of the furnace toward the firing door, f secure a circulation of over-oxidized copper on the top of the molten bath from the skimming door tow rd the opposite end of the furnace where the firing door is located, and by introducing an insufficiency of air at the firing door, I obtain slightly reducing conditions at that end of the furnace. As a result, I obtain a slow circulation of the charge of copper being fined, the copper at one end of the furnace being slightly over oxidized and slightly overheated compared with the re mainder of the copper in the bath, and tend ing to circulate toward the firing door, where the excess oxygen tends to be reducedand the molten and somewhat purified copper then tends to recirculate toward the other end of the furnace, thus cyclically becoming slightly modified periodically both toward oxidation and toward partial reduction.

As a means of saving still additional time in the fining operation, I find that in some cases I can begin the oxidation of the charge of copper one or more hours before all of the copper in the furnace has been melted,

or before all of the copper off the bottom by introducing my porous di scininating tip into the pool of molten copper as soon as it has reached siuficient depth in the furnace, and passing a gaseous reagent high in oxygen into the molten copper. It will be noted that it is possible to do this as a part of my improved process because of the heat which is produced at the point of reaction being very great, as compared with a very slight evolution of heat when ordinary air is introduced ii o the copper by the methods which have hitherto been employed.

'Although oxygen may appear to be a relatively expensive reagent to use in a commercial process such as the fining of copper, I have discovered that it is unnecessary to use pure oxygen, and that any enrichment of the air with oxygen to produce mixtures hav- 25% or more of oxygen by volume pro duce effects entirely disproportionate to the amount of oxygen actually introduced, so that the procedure, instead of being relatively wasteful, is actually more efficient and economical than the fining of copper by the methods now in universal use. Even by the use of highly concentrated oxygen costing at present from 20, to 2 per pound substantial economy can be shown under certain contions, but I do not .find'that the use of high concentrationsof oxygen are necessary, as a relatively slight disturbance of the equilibrium point of the present sluggish oxidation reaction may be obtained by the use of a relatively slight increase in the partial pressure ofthe exygen'supplied, and with a corresponding very great increase in the effects produced. It will of course be recognized that the efficient operation of my present invention is directly connected with the proper utilization of the oxygen supplied to the bath. Under the conditions of oxidation at present employed the fining of copper the actual efficiency of oxygen utilization is very low, and most of the air supplied to the fining furnace passes through the molten copper, or over its surface, without exercising any oxidizing effect whatever. By slightly increasing the partial pressure of oxygen, however, I have discovered that the reaction be tween oxygen and copper is so greatly speeded up as to represent in effect an entirely different order of magnitude, and that under the described conditions the fixation of oxygen is sufficiently complete under favorable conditions to represent an efficient commercial process, the oxidation. of the molten copper being preferably continued until more than one-half of one per cent of oxygen is present in the molten mixture, but the addition of oxygen being discontinued before the con centration of oxygen in the molten mixture is increased to one percent by weight. Under normal operations the introduction of sufficient oxygen to produce a concentration of seven tenths of one percent of oxygen gives satisfactory results, this corresponding to the presence of approximately 6.5% of cuprous oxide in the molten copper.

It will be evident that many modifications may be made in the practice of my invention its Within the limits of the disclosure as herein made, and that accordingly no limitations are to be placed upon my invention except such as are indicated in the appended claims.

I claim:

1. In the fining of copper the process which comprises simultaneously oxidizing and reducing a bath of molten copper by maintaining one or moreloca'lized zones of oxidation within the body of the moltencopper While maintaining reducing conditions at the surface of the molten copper, and producing circulation within the copper bath to continuously transfer copper from the oxidizing Zone to the reducing zone.

2. In the fining of copper the process which comprises maintaining .a mixture of molten copper and solid copper at :the melt ing pointofcopper While introducingagaseous reagent comprising :more than 25% Joy volume of oxygen into the molten phase of the mixture, until all ofrthe solid copper has been liquefied.

3. In .the .fining of :copper Jhe process which comprises maintaining a mixture of molten copper and solid @copper at the meltin g pointofcopper while introducing a ,gaseout reagent comprising more than 25% volume of oxygen into the molten phase of the mixture, until (all of the solid copper has been liquefied With the production of a mixture containing more than 0.5% by Weight of oxygen but less than 1% by Weight of oxygen.

Intestimonywhereof I hare hereunto subscribed my vname thisQTthday of December,

' VVALIER O. SNELLING. 

